I have seen both and BoxTone is by far the best of the two, whether it is worth the extra money is another thing that only you can decide. If budget is tight Zenprise is a very good product, I only wish they did a version for Domino as I was sold and ready to sign on the dotted line when they asked "I assume your Exchange, right?!"
No longer have 15 Domino 6.5 Servers
No longer have 6 BES 4.1.3 Servers
No longer have 2613 Users
But still have the 1 Constant Headache?!?!